This application claims the benefit of International Application No. PCT/EP98/01937, filed Mar. 31, 1998.
1. Field of the Invention
The present invention generally relates to a process for making a fluid-tight connection between a tube and a plate-shaped part. More particularly, this invention relates to a process that entails inserting one end of a tube into a correspondingly-sized hole in a plate-shaped part so that the tube end extends beyond the surface of the plate-shaped part, inserting a mandrel into the tube end, and then pressing the mandrel into the tube end while heating the mandrel to a temperature sufficient to soften and deform the tube end, thereby securing the tube to the plate-shaped part.
2. Description of the Prior Art
Such a process is described in EP-A-0 379 701.
There is a permanent need to secure leak proof and strong connections between tubes and plate-shaped parts such as those used in heat exchangers. As used herein, xe2x80x9cplate-shapedxe2x80x9d part meant to denote any object having at least one portion characterized by a relatively thin wall with a substantially flat surface. Examples are tubes with at least one substantially flat surface, such as headers in heat exchangers, box-like structures to be used as containers, such as gasoline tanks, etc.
It is known to use mechanical expansion of the tube end into the plate-shaped part in order to anchor the tube ends in the header plates. This is especially used in heaters and radiators where resilient gaskets may be used as well in order to obtain the required fluid tight connection. However such arrangements are not sufficiently secure for containment of the heat exchanging fluid such as used in air conditioning arrangements in which higher pressures and fluids having smaller molecules are used than in water-based domestic heaters and radiators. Furthermore, where the said heat exchanger tube is present in a hard temper, it becomes increasingly difficult to obtain a good seal by mechanical expansion alone due to the xe2x80x9cspring-backxe2x80x9d effect. There is also an increasing risk of splitting the tube if a too high expansion force is applied to such a hard tube.
It is therefor an object of the invention to provide a process for making a fluid tight connection between a tube and a plate-shaped member which has improved characteristics compared to the known processes.
This and other objects are achieved in a process according to the introductory part of the description in that a mandrel is inserted into said tube end, said mandrel comprising a first portion having a cross-section substantially corresponding to the inner cross-section of the tube, a second portion having an increased cross-section and a transition portion between these two portions, in that the tube is maintained in its inserted position while pressing the mandrel into the inserted end of the tube and heating, the mandrel being heated to a temperature at least sufficient to soften the tube material, and this situation is maintained during a time sufficient to deform the tube end on the plate-shaped part.
By using a mandrel according to the invention and applying heat to the mandrel, it becomes possible to provide a connection between the plate-shaped part and the tube which is partly obtained by deformation of the tube over the edge of the hole in the plate-shaped part. This can be achieved easily due to the local annealing of the tube end with minimal risk of splitting the tube and without waste energy by heating only the places where the heat is needed. In this way a fluid-tight and mechanically reliable connection is obtained between the tube and the plate-shaped part.
Preferably the tube and/or the plate-shaped part are made out of aluminum or an aluminum-alloy. This type of material is especially effective in heat transfer applications and can be easily deformed at lower temperatures. Due to the spring-back effects from the mechanical deformation, if might be necessary to seal the tube to the plate shaped member by a more secure method, such as by using a brazing connection.
Present brazing techniques for aluminum utilize Al:Si alloys for achieving a brazed joint, usually by use of clad sheet. This is a fairly high temperature operation, well above the temperature at which aluminum alloys become annealed and softened. Therefor, heating of the whole of the assembly plate-shaped part and tube to effect a braze would soften the tube and the part so that deformation of the assembly is likely to occur under normal service conditions where high internal pressures can be encountered.
Localized heating such as used when flame brazing is not feasible or is difficult to realize due to geometrical restrictions and the possibility of heat application outside the desired region.
According to the invention, use is made of the plate-shaped or extruded part which is at least partly coated with a brazing and/or flux material.
In this way it becomes possible to heat the assembled tube and plate-shaped part through the heated mandrel in order to obtain a brazed connection between the two parts to be joined.
If needed only the area around the hole of the plate-shaped or extruded part which needs to be connected to the tube may be provided with such a coating.
An advantageous coating material is zinc, allowing to reduce the temperature of the joining process substantially as zinc will melt and diffuse into the aluminum and form a reliable brazing material at low temperatures.
The invention also relates to a process for making a fluid-tight connection between a header and a number of aluminum tubes coated with brazing and/or flux material, for making a heat exchanger, wherein the header at least comprises a plate-shaped member.
Heat exchangers require a substantial number of connections to be made between a header and a number of tube ends in order to provide the flow channels. Obtaining a reliable fluid-tight connection between the header and the tube ends is still a cumbersome operation, and most failures in this type of heat exchanger occur at the connection between the tube ends and the header. This is partly due to the fact that joints place between the header and the tubes are difficult to reach as a result of the number of tubes and the fins already arranged around the tubes. This can result in unsound brazed joints.
It is therefor also an object of this invention to provide a process whereby the making of the connection between the header and the tubes is substantially improved and facilitated.
This object is obtained in that the connection between each tube and the plate-shaped member of the header is made in accordance with the invention and the plate-shaped member is subsequently modified into a header.
In this way it becomes possible to make a number of connections between tubes and the plate-shaped member of the header in one operation by using the required number of mandrels, and without overheating the already partly assembled heat exchanger, thereby avoiding unwanted deformation of the assembled unit.
The transformation of the plate-shaped member into a header can be easily done either by deformation of the plate-shaped member so that a closed tube-like header is obtained which can easily and reliably be made fluid-tight by brazing, as the joint can be made at an easily accessible place. Otherwise it is possible to make the header by adding another unit to the plate-shaped member and sealing the connection edges between the member and the added unit, thereby providing a tube-like header.
Other characteristics and advantages of the invention will become clear from the following description reference being made to the annexed drawings.